Known in the art is an internal combustion engine which arranges, in an engine exhaust passage, an NOx storage catalyst which stores NOx which is contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and which releases the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes rich, which arranges, in the engine exhaust passage upstream of the NOx storage catalyst, an oxidation catalyst which has an adsorption function, and which feeds hydrocarbons into the engine exhaust passage upstream of the oxidation catalyst to make the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst rich when releasing NOx from the NOx storage catalyst (for example, refer to Japanese Patent No. 3969450).
In this internal combustion engine, the hydrocarbons which are fed when releasing NOx from the NOx storage catalyst are made gaseous hydrocarbons at the oxidation catalyst, and the gaseous hydrocarbons are fed to the NOx storage catalyst. As a result, the NOx which is released from the NOx storage catalyst is reduced well.
Further, it has been proposed to estimate the NOx storage amount of the NOx storage catalyst and, when the estimated NOx storage amount has become a set amount and if the temperature of the NOx storage catalyst is a set temperature more, judge that the NOx should be released and make the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst rich. (for example, refer to Japanese Unexamined Patent Publication No. 2009-275631).